1. Field of Invention
The present invention relates to microwave circuits provided with dielectric resonators whose resonance frequency must be electronically controlled and in particular relates to a coupling network and a method for widening the varactor diode tuning band of microstrip-coupled dielectric resonators.
2. Background Information
There are microwave circuits provided with dielectric resonators (or simply DR) whose resonance frequency must be electronically controlled by means of varactor diodes, which could be required to feature a tuning band extending beyond the very narrow limits usually obtainable with conventional networks. This is the case of, e.g., microwave oscillators or filtering arrangements using microstrip-coupled dielectric resonators which need electrical tuning, or similar devices.
A conventional coupling network between a dielectric resonator and a varactor diode, both placed on the same face of a microstrip circuit, includes a length of transmission line which is terminated at one side only, by means of the varactor diode and near to which the resonator is fixed. The control voltage is applied to the varactor diode through a suitable RF decoupling network. The transmission line and varactor diode assembly is dimensioned in such a way as to resonate at about the nominal frequency of the dielectric resonator. During the circuit operation, the magnetic field lines of the resonator interlink with the transmission line. By varying the bias voltage of the varactor diode, the capacitance of the latter is modified and the change of the resonance frequency of the dielectric resonator is thus determined. Unfortunately the tuning band obtainable in the manner described above is very narrow and generally it does not exceed 0.1%-0.2% of the resonator nominal frequency.
Another known method of widening the relative band up to 0.5%-1.0% consists in applying a ferrite element on the dielectric resonator, which modifies the distribution of the magnetic field lines, to be tuned by means of an external magnetic field generated by an external current-carrying winding. Such a solution however is impractical and has several drawbacks, among which: i) implying an increase of size (because of the overall dimensions of the electromagnet structure); ii) a remarkable sensivity to external magnetic fields and the consequent need for magnetic shields; iii) microfonics; iv) high consumption due to the electromagnet bias current; and v) a significant slowness of response due to current driving (as it happens for YIG oscillators).